In conventional coaxial cables, a center conductor is surrounded by a dielectric medium which in turn is surrounded by an outer conductive shield serving as an outer conductor positioned generally coaxial with the center conductor. This outer conductor is conventionally formed by a braid of electrical wires and in some cables a second braided shield surrounds the first, and the composite outer conductor is called a double shield braid.
The diameter of the center conductor may be called "d", and the inside diameter of the outer conductor may be called "D". The characteristic impedance (sometimes called the "surge impedance") of a coaxial cable is a function of the "d" to "D" ratio, which is often expressed as "d/D" ratio , wherein "D" is concentric (coaxial) with "d".
Electrical losses which occur in transmitting a microwave signal, i.e., an electrical signal in the frequency range from about 0.1 GHz up to about 35 or more GHz, through a length of coaxial cable depend to a considerable degree upon the nature of the dielectric medium positioned in the region between the inner conductor and the outer conductor. In other words, the dielectric medium is located in the region between the dimensions d and D. Electrical losses which occur in transmitting an electrical signal through a length of coaxial cable are called "attenuation losses".
In U.S. Pat. No. 4,626,810, issued Dec. 2, 1986, to my son, Arthur C. Nixon, is described the desirability for achieving reduced attenuation losses by using low density dielectric medium containing numerous tiny air pockets, for example such as low density (sometimes called "expanded") PTFE dielectric material. In that patent are described and claimed low attenuation microwave coaxial cables for operation in a GHz range, for example up to about 18 GHz, having an arrangement of such low density dielectric material positioned between the center conductor and the outer conductor.
Since the characteristic impedance of a coaxial cable depends upon the d/D ratio of the cable, it will be appreciated that mechanical stresses imposed upon a coaxial cable which cause deformations or distortions away from true concentricity of D relative to d, for example such as caused by squeezing, ovalizing, flattening or squashing of the cable under mechanical loading or bending will cause localized changes or variations in d/D ratio and hence will cause localized changes, variations or discontinuities in the characteristic impedance of the cable. Moreover, it will further be appreciated that a low density dielectric medium having numerous tiny air pockets therein inherently provides less mechanical support for the outer conductor to resist crushing forces than the support provided by high density solid dielectric medium of the same material. Consequently, a microwave coaxial cable having low density dielectric medium providing enhanced electrical performance such as disclosed and claimed in said '810 Patent is likely to be more susceptible to deformations or distortions away from true concentricity for a given mechanical loading or bending than one having a high density dielectric medium, because the outer conductor receives less internal support from a low density dielectric medium.
Such localized changes in characteristic impedance within a coaxial cable due to mechanical loading or bending distortions or deviations in d/D ratio are undesirable because they produce localized impedance mismatching within the cable causing backward reflections of electric signals. The original signals were being propagated in a so-called "forward" direction through the length of coaxial cable, and reflected signals due to impedance mismatching become propagated in a "backward" direction through the same length of cable. The resultant interactions of the forwardly and backwardly propagating signals produce "standing waves" within the coaxial cable. Not only do reflections undesirably weaken (attenuate) the desired forward-going signal but standing waves undesirably increase electrical losses within the cable.
A measurement of the magnitude of standing waves within a microwave cable is the Voltage Standing Wave Ratio (VSWR). In a perfectly uniform and stable coaxial cable having a perfectly impedance matched termination, the VSWR measurement would be 1.00 throughout a desired operating range of frequencies. This optimum VSWR of 1.00 throughout a desired operating range of frequencies in my experience has not been achieved in any commercially available coaxial cable harness.
As further background, it is noted that my U.S. Pat. No. 4,408,089, issued Oct. 4, 1983, discloses an extremely low attenuation low radiation loss flexible coaxial cable for handling microwave energy in the GHz frequency range. In that patent a flexible dielectric medium which covered a center conductor was surrounded by a plurality of longitudinal, parallel, contiguous conductive strands with a slight helical lay which in turn were surrounded by means to hold them in place, including an outer jacket of flexible impermeable material such as plastic. The coaxial cable of that patent provides superior performance with respect to attenuation loss, leakage, and other properties for microwave signals as compared with conventional coaxial cables having braided outer conductors including those having a double shield braid. Each of the contiguous conductive strands is smooth silver plated. All of these strands extend longitudinally of the cable, and they are sufficiently numerous for forming at least two full layers of these strands surrounding the dielectric medium. The inner layer of strands is contiguous to the dielectric medium, and the next layer comprises strands nesting in the valleys defined by the respective neighboring strands of the inner layer. These parallel strands are tightly secured in place retained tightly embraced against the dielectric medium and against each other by a continuous, uniform, tightly fitting, squeezing wrapping serving of strong, fine filaments or fibers which are wound tightly around the longitudinally extending contiguous conductive strands of the outer conductor.
The '089 Patent specifies a particular example in which the longitudinally extending conductive wire strands had a diameter of about 0.004 of an inch, and the wrapping serving was applied directly over the wire strands. This wrapping serving comprised eight multi-filament fiber glass threads, each thread being impregnated with FEP (fluorinated ethylene propylene) and having a fiber glass thread diameter of approximately 0.004 of an inch. An outer jacket of flexible impermeable plastic surrounded the wrapping serving for protecting the coaxial cable.
In introductory discussion in the '089 patent preceding the above-described particular example, it is stated that in order to retain the conductive strands of the outer conductor firmly pressed in adjacent relationship one to another and tightly embraced against the outside of the dielectric medium, there is a continuous, uniform, tightly fitting wrapping or serving. This serving is formed of strong stranded or ribbon material capable of withstanding the heat curing temperature of the plastic jacket. The patent states that, for example, this serving is formed of thread, plastic ribbon, metallic ribbon, or wire strands or metallized plastic ribbon, e.g. metallized Mylar. The metallic ribbon or metallized Mylar is employed in order to provide additional shielding against external or internal radiation, if desired, in special applications requiring unusually extreme isolation of the signal being carried in the cable. The '089 Patent explains that in the embodiment being shown, the serving is formed by threads each having a diameter comparable with the diameter of the parallel conductive strands of the outer conductor, namely 0.004 of an inch (American Wire Gage 38). Each thread contains multiple fine filaments, for example glass filaments, with the thread being impregnated with FEP (fluorinated ethylene propylene) or a thread of Nextel filaments (obtainable commercially from 3M Company in Minneapolis, Minn.), with the thread being impregnated with PTFE (polytetrafluoroethylene).
There is no other purpose stated in the '089 Patent for the wrapping serving applied directly over the longitudinally extending contiguous conductive strands of the outer conductor, except to retain the parallel conductive strands of the outer conductor firmly pressed in adjacent relationship one to another and tightly embraced against the dielectric medium.
These contiguous conductive strands comprising the outer conductor in the microwave coaxial cables described in my '089 Patent are silver-coated for increasing surface conductivity of the outer conductor. Arthur Nixon's '810 Patent discloses that incorporation of the low density dielectric medium arrangement described and claimed within a microwave coaxial cable of the structure as disclosed and claimed in the '089 Patent, enhances performance by further reducing attenuation losses.
For many years the coaxial cable industry has protected coaxial cables against crushing or mechanical distortion under squeezing or bending loads by inserting the coaxial cable endwise through a length of flexible conduit armor surrounding the whole cable. This flexible conduit armor consists of a single strip of stainless steel wound helically with each successive turn of the helix being convoluted, so as to interlock with the preceding turn in a manner similar to the construction of flexible steel armor around electrical "BX" cable used in homes and commercial structures for carrying 60 Hz AC electrical power. Among the problems of using such flexible conduit armor placed around the outside of a whole coaxial cable are that it adds about 0.150 of an inch to the outside diameter of the assembly of cable plus armor and it adds considerable size, mass and weight to the assembly as a whole. Further, such flexible conduit armor restricts the ability to bend coaxial cable. Attempts to bend such flexible conduit armor into a circular arc having a bend radius smaller than about 1.2 to about 1.5 inches can split open and dislodge the interlocking convolutions of the stainless steel strip, thereby destroying the armor and creating jagged, dangerous or unsafe sharp edges exposed on the split-apart convolutions of the conduit armor.
Another problem from using such flexible conduit armor around the outside of a whole coaxial cable having construction as described and claimed in the C. E. Nixon '089 Patent incorporating low density dielectric medium as described and claimed in the A. C. Nixon '810 Patent (hereinafter called "the '089+'810 microwave coaxial cable") is that in my experience the cable with its external armor can be bent repeatedly to a radius of about two inches and straightened only about 38 to about 40 times in testing, before breakage occurs; whereas the '089+'810 microwave coaxial cable incorporating the internal ruggedization of the present invention in a preferred form can be bent repeatedly to a radius of about two inches and straightened at least 1,000 times without breaking.